If you're not used to GitHub (or Git!) then getting code out of GitHub can be pretty confusing.
There are 2 ways to download an entire project. You can either download a ZIP or use Git to clone the project.
Whichever method you use, you need to use the find the "Code" button. This is a big green button that appears on the project's home page.
If you're not on the home page you can use any of the links that
contain the project name. That's teensy4_i2c in these examples.
Click the green "Code" button. Select "Download ZIP" to download a zip file containing the project.
If you're using git, you can get the git repository URL from the drop down menu.
"Branches" contain different versions of the code. If you want to download a ZIP for a differnt branch then use the drop down on the left hand side of the screen to change branch before clicking the "Code" button. The example above is showing the "master" branch.